As portable electric devices are more and more popular nowadays, a switching power has become the primary power scheme in these portable electric devices. Currently, a low-power AC/DC (alternating current/direct current) switching power always applies a primary control to replace an expensive secondary control (comprising a TL431 and an optical coupler). For example, FIG. 1 illustrates an adapter of a typical primary control switching power, in which after passing a full wave rectifier (i.e. the rectifier bridge consisting of diodes D1, D2, D4, and D5) and a π type filter circuit consisting of capacitors C2, C3 and an inductor L1, an AC voltage is converted into a high-voltage DC voltage to supply a start voltage for the control chip IC1 and to provide energy for the primary coil power loop after the chip IC1 is turned on. The PWM (Pulse Width Modulation) chip IC1 controls a turn-on time of a switching tube such as MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor, MOS tube for short) by sampling a voltage dividing signal of an auxiliary winding (the feedback voltage dividing network consisting of resistors R2, R3) and processing the voltage dividing signal by EA (Error Amplifier) in the interior of the PWM chip IC1 to generate a control signal, so as to adjust an input power to accommodate a change in the load. A resistor R4 is used for detecting a peak current of the primary coil of the transformer, D3 and C1 form a secondary rectifier filter network, and R1 is a fictitious load resistor. A conventional control chip adopts a control mode of PWM or PFM (Pulse Frequency Modulation), which provides good constant voltage and constant current output properties, but has limitations in the efficiency and EMI (Electro-Magnetic Interference). The chip IC1 comprises pins as follows:
VDD—external power terminal;
GND—ground terminal;
Isense—terminal for detecting peak current;
Vsense—terminal for feeding back output voltage;
OUT—output driving terminal.
With more and more focus on the energy and the environment, currently there are higher requirements of the switching power for the efficiency and EMI. For example, as shown in FIG. 2, a QR (Quasi-Resonant) circuit comprises a buffer capacitor Coss, also called resonant capacitor which mainly comprises an output capacitor of an MOS tube M0 and a parasitic capacitor of the transformer. When the MOS tube M0 is turned on (Ton), the input voltage VIN is applied to the primary coil Lm, and a current Ids of the MOS tube M0 increases linearly from zero to a peak value Ipk. During this time period (Ton), the energy is stored in the primary coil Lm and represented by a formula of (Lm*Ipk*Ipk)/2. When the MOS tube M0 is turned off, the energy stored in the primary coil Lm causes a rectifier diode D3 at the secondary output terminal to be turned on. During the time period (Tds) when the diode D3 is on, the output voltage Vo is applied to the secondary coil, a current of the rectifier diode D3 decreases linearly from a peak value Ipk*Np/Ns, and the input voltage VIN and a voltage Vo*Np/Ns fed back to the primary coil from the secondary coil overlap onto the MOS tube M0, where Np is a number of turns of the primary coil of the transformer, and Ns is a number of turns of the secondary coil of the transformer.
In conjunction with FIG. 2, FIG. 3 illustrates waves of each node during the operation process of the quasi-resonant circuit, in which Ip is a wave of a current passing the primary MOS tube M0, Is is a wave of a current passing the secondary diode D3, and VDS is a wave of a voltage between two terminals of the MOS tube M0. When the current of the diode decreases to zero, the voltage VDS of the MOS tube M0 starts to resonate with an amplitude of Vo*Np/Ns by the primary coil Lm and the output capacitor Coss of the MOS tube M0. In that way, a switching loss caused by the capacitance between a drain and a source can be reduced, which is also named ZVS (Zero Voltage Switch) or LVS (Low Voltage Switch). At the same time, smaller voltage change rate dv/dt also improves an effect of EMI.
As described above, the conventional switching power has the following disadvantages: 1) the ZVS or LVS cannot be achieved by the conventional PWM, so the switching loss is huge and the EMI effect is poor; 2) although QR control may realize the ZVS or LVS, when an output load is reduced or an input voltage is increased, turn-on time Ton of the MOS tube may reduce which leads to an increase in a switching frequency, thus causing problems like a significant switching loss, an intermittently switching and a noise.